Skidder machines conventionally take the form of articulated frame, rubber-tire tractors. A front section supported by front drive wheels includes the engine and the operators' cab. A rear section supported by rear drive wheels is connected to the front section on a vertical axis pivot so as to be articulatable relative to the front section. The front and rear wheels are driven by conventional drive trains from the engine through a gear box controlled by the operator. Steering is accomplished by varying the angular relationship between the front and rear sections with a hydraulic cylinder steering system
Timber Jack, Caterpillar and John Deere & Co., as well as others, manufacture skidder machines of the aforedescribed nature. Caterpillar also manufactures track skidder machines, i.e., machines where continuous drive tracks replace the four wheels.
Regardless of whether the skidder machine is wheel or track driven, loggers have found skidders to be the most versatile and cost-effective way to bring in felled trees, terrain and other conditions permitting. A hydraulically controlled boom extending rearwardly from the rear section of the skidder machine grasps a tree or a bunch of trees at a lower end with a grapple or cable loop. The boom is actuated to raise that end of the bundled trees, for example, off the ground and hold them suspended in the air behind the rear wheels of the skidder machine while the machine drags or "skids" the trees out of the forest to waiting transport.
Conventional machines which are constructed and operate in the aforedescribed manner are versatile and cost-effective, as has been pointed out. However, because the trees are lifted by the grapple, for example, at their lower ends and carried behind the rear wheels, both the rear wheels and the tops of the bundled trees caused substantial ground disturbance. The weight of the lifted trees at their lower ends and the counter-weight effect of the machines front end cause the rear wheels to rut and shear soil as they exert traction. The tops of the trees dragged along the ground also cause substantial ground disturbance. In addition, the trees frequently suffer damage as they are skidded out of the logging area.